For the sterilizing of stacked containers, such as cups for the packaging of sterile materials like comestibles (see German patent 2,310,661), it is common to separate the containers of the stack for sterilization and to enable the sterilizing agent to contact all surfaces of the container. In this system, gripper devices on a circulating conveyor chain engage the containers and move them apart so that a gap can be provided between them. The gap must be sufficient to permit the sterilizing agent, for example, a 35% aqueous H.sub.2 O.sub.2 solution, to be sprayed from the spray nozzles along the inner and outer walls of the container to completely wet the latter with the hydrogen peroxide solution.
After spraying with the sterilizing agent, the containers are treated in a drying chamber downstream of the sterilizing chamber, to hot air. The hot air vaporizes residues of the sterilizing agent and the heat causes thermal decomposition of the hydrogen peroxide into water and oxygen, thereby eliminating the sterilizing agent from the containers.
In the packing of sterilized products in germ-free packaging under aseptic conditions, it is important that the sterilizing of the packaging materials, for example, plastic cups which can receive liquid (flowable) or pasty substances such as milk, yogurt, sour cream, prepared soups and the like, be effective to remove or destroy all pathogenic microorganisms and, further, that the film of H.sub.2 O.sub.2 which is applied to the inner and outer surfaces of the container be completely removed without thermal damage to the container.
The sterilization with hydrogen peroxide utilizes the strongly oxidizing effect of the concentrated hydrogen peroxide solution (35% concentration) at an elevated temperature (60.degree. C. to 80.degree. C.) which can result in a destruction of even resistant microorganisms, such as bacillus spores, with a contact time or residence time of the order of seconds. For the aseptic filling of neutral substances in packages having a volume of up to two liters, a reduction rate of resistant microorganisms, for example the bacillus subtiliis spores, of at least 4 orders of magnitude has been found to be necessary.
It is known that there is a relationship between the microorganism killing effect of hydrogen peroxide and its concentration and the temperature of the H.sub.2 O.sub.2 solution. The microbicidal effect of hydrogen peroxide increases with increasing concentration. The germ-killing effect of the hydrogen peroxide solution is also improved with increasing temperature.
A measure of the microbicidal effect is the so-called D-value (i.e. the decimal reduction time). This value is the time in minutes which results in a kill of 90% of the microbe population at a given constant temperature. A killing of refractory spore formers in the range of seconds, as is required in aseptic packaging machinery, is only possible with H.sub.2 O.sub.2 concentrations of about 30% and more at a minimum temperature of 80.degree. C. to 90.degree. C.
International regulations and those of the Food and Drug Administration have established strict guidelines with respect to the sterilizing effect which is required for foodstuff packaging and the like.
To satisfy these requirements, efforts have been made to increase the temperature of the H.sub.2 O.sub.2 solution which has been used. While this has been found to be effective in the treatment of foil-packaging materials, the sterilization of containers, especially plastic cups, has not been found to be as simple. Prefabricated plastic cups require various stacking elements, such as ribs, undercuts, raised bottoms or the like for their stability and separability. These regions require, for an effective microbial kill effect, a relatively-long contact time and a higher thermal efficiency for removal of the traces of the sterilizing agent than thin wall, uniform thickness packaging materials. The packaging cups with varying wall thicknesses are also more sensitive to thermal deformation in many instances.
While it has been recognized that the degree of microorganism contamination of a packaging material determines, by and large, the conditions under which sterilization must be effected to obtain the requisite sterility of the packaging material for the sterile packaging of foods and the like, it has also been found that the plastic cup packaging with variable wall thickness containers and intricate configurations of the cup are most susceptible to high degrees of contamination and thus have, in the past, generally required long contact times with the sterilizing agent, larger amounts of higher concentration sterilizing agents and longer periods of treatment with hot air for removal of the sterilizing agent.